High temperature structural material and method of producing same



The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to aluminum base metallurgical compositions, to lightweight structural materials produced therefrom and to a method of producing such materials from such compositions.

More particularly this invention relates to aluminum base powder metallurgical compositions comprising aluminum and/or alloys of aluminum mixed with certain oxygen bearing compounds of those metals having high melting points and good solid solubility in aluminum, to lightweight structural materials prepared from such compositions having unusual strength and hardness at elevated temperatures and to a method of producing such materials from such compositions by the utilization of certain techniques of powder metallurgy.

This application is a continuation-in-part of my application Serial No. 479,874, filed January 4, 1955, titled High Temperature Structural Material and Method of Producing Same, now U. S. Patent No. 2,840,891.

In the prior art the presence of substantial percentages of metal oxides, mixed metal oxides, inorganic compounds of metal oxides and other inorganic non-metallics have always been considered to be detrimental to the useful properties of the base metal or alloys of such metal containing such substances. Contrary to the teachings of the prior art, it has been found that the proper amount and distribution of certain of these substances in metals and metal alloys leads to enhanced properties of these substances.

For example, structural materials possessing unusual and desirable properties may be produced by powder metallurgical processes from compositions of aluminum and aluminum alloy powders containing in finely divided form, oxides of those metals having high melting points and good solid solubility in aluminum where such oxides contain no non-metal other than oxygen. The structural materials produced therefrom in accordance with this invention are characterized by relatively light weight and greatly improved strength and hardness at elevated temperatures up to about the melting point of aluminum.

Accordingly, it is the principal object of this invention to produce aluminum base structural material possessing lightweight and greatly improved hardness and strength at temperatures up to about the melting point of alul; minum.

Another object of the invention is to produce lightweight structural material characterized by unusual strength and hardness at elevated temperatures from compositions comprising aluminum and/or aluminum alloy powder and certain finely divided oxygen bearing compounds of high melting metals.

A further object is to provide the metallurgical powder composition from which the before mentioned structural material may be produced.

A still further object is to provide a method by which the before mentioned structural materials may beproduced from the before mentioned compositions.

Still further objects and the attendant advantages of the invention will be apparent to those skilled in the art from the more detailed description set forth below, it being understood that the following detailed description ate r,

ice

is given by way of illustration and explanation only, and not by way of limitation, since various changes therein may be made by those skilled in the art without departing from the spirit and scope of the invention.

The metallurgical composition of the invention is made up principally of a metallic base and a metallic oxide additive. The metallic base may be finely divided al minum powder or aluminum alloy powder or mixtures of such powders. Flaked aluminum and aluminum alloy powders of the size which will pass a 325 mesh screen have been found to give the best results when used in the powder compositions of the invention. Less finely divided powders result in compacts which are not as hard at elevated temperatures. Atomized aluminum and aluminum alloy powders may also be employed if desired.

The alloys of aluminum which have been found most useful in the compositions of the invention are alloys of aluminum with those metals which are known to those skilled in the art as strengthening metals for aluminum and aluminum alloys. Examples of such alloys are alloys of aluminum with iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron and the rare earth elements.

It has been found that the above strengthening metals may also be added in finely divided form directly to the aluminum and aluminum alloy powders and may constitute up to about 25 percent of the metallic base with excellent results.

The metallic oxides which have been found useful in the metallurgical powder compositions of the invention are the oxides of those metals which when liberated have a high melting point and good solid solubility in aluminum, which oxides do not contain any non-metals other than oxygen and which are stable and not appreciably reactive with aluminum and oxygen at temperatures up to about the melting point of aluminum.

It has been found that if the metal can be liberated from the oxygen bearing compound and dissolved in thematrix of aluminum, the resulting material has considerably greater strength than if the metal is present as part of an intermetallic compound. Therefore, the best compounds for use as additives in the metallurgical compositions of the invention are the oxides which are reducible by aluminum, have low heats of formation and which'upon reduction liberate a metal having a high melting point and good solid solubility in the aluminum base. These metals are boron, tungsten, thorium, iridium, tantalum, molybdenum and niobium. Oxides of the metals and mixtures of these oxides have been found to produce enhanced properties in products made from compositions containing from about 1 percent to about 25 percent by weight of the composition. The best results were obtained where the' composition contained from about 5 percent to about 15 percent by weight of the oxide.

It is highly desirable that the oxides be in a finely divided state'to promote homogeneity. An oxide particle size of 5-50 microns has been found to give good results.

The structural material of the invention is produced from compositions such as are described above by means of the following process:

(1) The powder composition is first prepared by mixing the metallic base powder with the metal oxide powder. The powder composition is preferably milled for a period of from fifteen minutes to one hour in a ball mill with sufiicient wetting agent such as xylol to form a soft paste. 7

(2) The powder composition is then presintered at a temperature of from about 700 F. to about 1100 F. but not above the melting point of aluminum under nonoxidizing conditions to remove the lubricants therefrom.

(3) The presintered powder composition is then cooled to room temperature before exposure to air.

(4) The powder composition treated in accordance with steps 1-3 above is then subjected to suitable techniques of powder metallurgy to produce the desired forms.

The purpose. of the presintering step is to remove the lubricant incorporated into the powder composition during the mixing step. Any satisfactory lubricant such as xylol, stearates or alcohols may be employed for this purpose.

A'study of'pr'esinteringconditions shows that, in most cases, a satisfactory presinter is obtained after one hour at 800 F. in an argon atmosphere. However, satisfactory presinters may also be obtained, depending on the characteristics of the particular composition and the lubricant employed, after a period of time varying from about one-half to about time hours at a temperature ranging from about 700 F. up to 'about the melting point of aluminum under any inert or non-oxidizing atmosphere.

It is important that during this presintering step that oxidation of the powder composition be substantially prevented in order that the product maybe substantially metallic as contrasted with conventional porous ceramics having low physical strength such as areproduced by the process disclosed in the United States Patent No. 2,568,- 157 to J. M. Lepp et al. The presinte'ring step in accordance with the present invention may take place under any inert atmosphere or non-oxidizing condition including vacuum and argon, or helium atmospheres. Control of the atmosphere during this 'first'heat treatment or presintering step is of the utmost'importance'since it is during this treatment 'that the metallic quality of the powder composition wouid be destroyedmost rapidly.

The metallurgical powder compositions prepared in accordance with steps 1-3 ofthe process described above may be processed into bodies of'desired size and shape by any suitable powder metallurgical method. For example, depending on the characteristicsof theparticular composition employed and the product desired, cold pressing or hot pressing alone may 'be sufiicient. Cold pressing may be combined with sintering or sintering with hot pressing, or a combination of thefo'regoing may be employed prior to mechanical deformation. Forging, rolling, extruding or casting may be employed either immediately after presintering or after'the sintering and pressing operations. Where 'sin'tering is found desirable, the composition should be pressedprior' to sintering. When this procedure is followed, sintering may proceed iri any desired atmosphere provided the sinter is not unduly prolonged as will be understood by those skilled in the art.

A study of pressing pressures for the mechanical or working processing of the composition described herein shows that in most instances a'satisfact'oly pressure is about 40 tons per sq. in. Pressures 'r'anging from 30 to 75 tons per sq. in. may be'employed successfully where the base metal of the composition containse 'substantial proportion of a metal other than aluminum but pressures less than 40 tons per sq. in. tend to produce unsatisfactory compacts in some instances while higher pressures tend to cause welding of the aluminum to the walls of the die and eventually result in the fracturing of'the die as well as the cracking of the specimen upon ejection from the die. However, regardless of the pressure employed, the temperature, at which the mechanical working and processing is being carried out, should not exceed the melting point of aluminum since, if the temperature goes substantially above that point, the physical properties of the product will suffer.

The following specific example of the preparation of a metallurgical powder composition and the manufacture offorms therefrom in accordance with-the invention is given by way of illustration. Howeve'nit'will be appreciated that the invention is not limited to the specific composition described and that other well known powder metallurgical techniques may be used to form larger or more complicated shapes from any suitable powder composition chosen from those previously described.

EXAMPLE A'dry powder composition was prepared from 95 percent by weight of low lubricant fiakealuminum powder and 5 percent by weight of molybdenum sesquioxide (M0 0 The aluminum base powder was of such a sizeas to pass a 325 mesh screen. The molybdenum sesquioxide had an average particle size of about 50 microns.

The above prepared powder composition 'was milled for about one hour in a ball mill with sufiici'ent xylol (60 cc.) to form a soft paste. The powder composition was then placed in an aluminum boat and presintered for about one hour at about 800 F. under an inert (argon) atmosphere. The presintered composition was then cooled to room temperature before exposure to air.

Compacts were then prepared from the presintered powder composition using a double action die at a pressure of about 40 tons per sq. in. The compacts were then sintered at a temperature of about 1000 F. for one hour under an inert (argon) atmosphere followed by cooling to room temperature and exposure to air.

It was found that compacts harder at room temperature were also harder at elevated temperatures. Accordingly, all compacts were repressed after sintering, by the process known as coining, in the same die used for making the compacts.

Hot hardness testing of the products was chosen be cause of the well known correlation between hot hardness, creep and creep-rupture strength. Compacts prepared as described above'were hot tested using a Rockwell Superficial Hardness Tester with a 15 kg. load and a 4; steel ball .indenter (Rockwell W Scale). The anvils were cylindrical stainless steel blocks 3"in height and 2 /2" in diameter with cylindrical cavities in the top. A larger anvil was used to support the stainless steel block with a Mr" sheet of transite interposed be tween the larger anvil and the stainless steel block.

The stainless steel blocks with a specimen compact in V Table I HOT HARDNESS OF ALUMINUM BASE POWDER PRODUCTS 7 Rockwell Superficial Hardness Number, Riv/l8 dia. steel Percent ball15 kg. load Additive to Al Powder As Sin- -At' At At At Pressed tered, coined ,1;000 1,100 1,150 1,200

F. F. F. F.

Pure AL--- 88 89 93 6G. 50' 6% M0 0 83 89 '65 62 5% ,Ti0 86 88 '67 56 5% V O5 "87' 84 89 63 48 5% ZrO 85 84 88 64. 55 5% Ce0 88 87 88 69 V 50 5% LaO--- 85 86' 88 66 46 5% PrO 88 85 90 70 52 5% Mo 87 v p Table II COMPRESSIVE STRENGTH OF ALUMINUM BASE POWDER PRODUCTS Compressive Additive Rw at 1,120 Strength,

F. p.s.i. at

6% M920 36 54.3 T1 g 32 56.6 0 V 37 52.6

5% Zl'Og 42 53.4

5% CeO- 25 53.4

6% PrO 25 524 Similar tests were run using the oxides of the high melting point metals having good solid solubility in aluminum listed above and with various alloys of aluminum with comparable results.

These results clearly demonstrate that products prepared from the powder compositions of the invention prepared in accordance with the process disclosed have a hardness markedly superior to aluminum and to products prepared from aluminum base compositions with oxides of high melting points which do not have good solid solubility with aluminum and have compressive strengths comparable to the best of the latter products. Regardless of the final form the structural material prepared in accordance with the invention is characterized by light weight, compact relatively non-porous forms of unusually high strength and hardness at elevated temperatures up to about the melting point of aluminum as distinguished from conventional porous ceramics having low physical strength.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practised otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A metallurgical composition consisting essentially of a finely divided mixture of at least 50 percent by weight of a metallic base selected from the group consisting of aluminum, alloys of aluminum with strengthening metals and mixtures of said alloys with each other and with aluminum, up to about 25 percent by Weight of a strengthening metal selected from the group consisting of iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron, the rare earth elements and mixtures of said metals, and from about 5 to about percent by weight of an oxygen bearing compound reducible by aluminum and selected from the group consisting of the oxides of boron, tungsten, thorium, iridium, tantalum, molybdenum and niobium which contain no non-metals other than oxygen and mixtures of said oxides.

2. The metallurgical composition of claim 1 wherein the oxygen bearing compound is molybdenum sesquioxide.

3. The structural material formed by mechanically 6 working under pressure from a metallurgical composition consisting essentially of a finely divided mixture of at least 50 percent by weight aluminum, up to about 25 percent by weight of a strengthening metal selected from the group consisting of iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron, the rare earth elements and mixtures of said metals, and from about 5 to 15 percent by weight of an oxygen bearing compound reducible by aluminum and selected from the group consisting of the oxides of boron, tungsten, thorium, iridium, tantalum, molybdenum and niobium which contain no non-metals other than oxygen and mixtures of said oxides, said composition having been heated under non-oxidizing conditions to a temperature between about 800 F. to about the melting point of aluminum.

4. The method of producing a structural material capable of unusual strength and hardness up to about the melting point of aluminum by heating under non-oxidizing conditions to a temperature in the range from about 700 F. to about the melting point of aluminum, a metallurgical composition consisting essentially of a finely divided mixture of at least 50 percent by weight aluminum, up to about 25 percent by weight of a strengthening metal selected from the group consisting of iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron, the rare earth elements and mixtures of said metals, and from about 5 to about 15 percent by weight of an oxygen bearing compound reducible by aluminum at temperatures in said range and selected from the group consisting of the oxides of boron, tungsten, thorium, iridium, tantalum, molybdenum and niobium which contain no non-metals other than oxygen and mixtures of said oxides and mechanically working said composition under pressure to form said material.

5. The process of claim 4 wherein the oxygen bearing compound is molybdenum sesquioxide.

6. A metallurgical composition consisting essentially of a finely divided mixture of at least 50 percent by weight aluminum, up to about 25 percent by weight of a strengthening metal selected from the group consisting of iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron, the rare earth elements and mixtures of said metals, and from about 1 to about 25 percent by weight of an oxygen bearing compound reducible by aluminum and selected from the group consisting of the oxides of boron, tungsten, thorium, iridium, tantalum, molybdenum and niobium which contain no non-metals other than oxygen and mixtures of said oxides.

7. The structural material formed by mechanically working under pressure a metallurgical composition consisting essentially of a finely divided mixture of at least 50 percent by weight aluminum, up to about 25 percent by Weight of a strengthening metal selected from the group consisting of iron, molybdenum, vanadium, titanium, beryllium, iridium, thorium, tantalum, niobium, chromium, manganese, nickel, cobalt, tungsten, boron, the rare earth elements and mixtures of said metals, and from about 5 to 15 percent by weight of molybdenum sesquioxide containing no non-metals other than oxygen, said composition having been heated under non-oxidizing conditions to a temperature between 800 F. to about the melting point of aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 2,840,891 Nachtman July 1, 1958 

1. A METALLURGICAL COMPOSITION CONSISTING ESSENTIALLY OF A FINELY DIVIDED MIXTURE OF AT LEAST 50 PERCENT BY WEIGHT OF A METALLIC BASE SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, ALLOYS OF ALUMINUM WITH STRENGTHENING METALS AND MIXTURES OF SAID ALLOYS WITH EACH OTHER AND WITH ALUMINUM, UP TO ABOUT 25 PERCENT BY WEIGHT OF A STRENGTHENING METAL SELECTED FROM THE GROUP CONSISTING OF IRON, MOLYBDENUM, VANADIUM, TITANIUM, BERYLLIUM, IRIDIUM, THORIUM, TANTALUM, NIOBIUM, CHROMIUM, MANGANESE, NICKEL, COBALT, TUNGSTEN, BORON, THE RARE EARTH ELEMENTS AND MIXTURES OF SAID METALS, AND FROM ABOUT 5 TO ABOUT 15 PERCENT BY WEIGHT OF AN OXYGEN BEARING COMPOUND REDUCIBLE BY ALUMINUM AND SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OF BORON, TUNGSTEN, THORIUM, IRIDIUM, TANTALUM, MOLYBDENUM AND NIOBIUM WHICH CONTAIN NO NON-METALS OTHER THAN OXYGEN AND MIXTURES OF SAID OXIDES. 